The invention relates to a device for projecting television pictures onto a projection screen by using three television picture tubes arranged side by side. Each tube has a display window having a display screen provided on the inside thereof (a phosphor layer on the vacuum side coated with a thin Al-film is common practice) and a light transmissive element arranged at a distance in front of the display window, which element together with the display window bounds a flow space for passing through a coolant. The flow spaces are in open communication with a liquid circuit for removing the heated coolant from and applying cooled liquid to the flow spaces. The light-transmissive element may be a window or a part of a lens system within the scope of the invention
A device having three juxtaposed television picture tubes is known from British Patent Specification 2,131,702 in which the flow spaces are arranged in series and in which a pump is used for the coolant. Chambers constituting the connection between the successive flow spaces are located between the juxtaposed television picture tubes This system is suitable for an energy dissipation of 60 to 80 watts per tube and has a satisfactory operation.
For many conventional uses, however, such a construction is too expensive whilst a considerably smaller heat dissipation can suffice. In the present-day projectors for consumer uses an energy dissipation of 10 to 20 W per tube is common practice.
A device suitable for a smaller heat dissipation up to 25 to 30 W per tube is described in European Patent Application 0,162,971, to which U.S. application Ser. No. 738,199 corresponds. This application describes a free convection cooling in which a closed liquid cooling system is provided around the front side of the tube. The coolant (lows upwards in front of the tube and flows back via a passage provided near the upper side of the tube and via further passages provided along the ends of the tube to a passage arranged under the tube from which the liquid can flow upwards again. The air circulating freely around the cooling system ensures a sufficient cooling of the liquid flowing through the passages from the upper side of the tube to the lower side of the tube. For a satisfactory operation it is required that the passages have a sufficiently large cross-section. This implies that the distance between the juxtaposed tubes is influenced to a considerable extent by the cross-section of the passages for the flow of the coolant extending along the sides of the tubes.
In connection with an optimum arrangement of the tubes with a view to obtaining the smallest possible distortions in the image on the projection screen it is required to have the smallest possible angles between the centre lines of the tubes among themselves and it is therefore desirable to arrange the tubes as closely together as possible.